52 twin H issues.

dlong59

This car has sat fo some years with lite use for the last 20. Car has 42k on odo original paint interior etc. twin h manifold was ate up so found a decent replacement. Rebuilt cabs extremely throughly, ignition and fuel system ( tank pump lines). Put a block off plate between intake and exhaust manifold due to suspect Welch plugs at bottom of intake. Car ran awesome! Carbs synced up nicely etc.   the car sat for couple months then went to our paint guy to do his best to bring back the original paint. Then sat for another month or so. Now we have a horrible idle, very rough. Pulled carbs down they are good and synced as close to perfect as we can get. High end runs great. Compression is close to 100 on all cylinders except the two  center cylinders (3&6). Going to pull the head I guess. Just have a hard time believing after running so well from its long hibernation this starts out of no where. Any dumb ideas I’m not seeing?!?!?

Ol racer

The two center cyl's or #3 & #6? Either way check the sparkplug Electrode color & wires first. Ive blocked (seperated) the Intake from Exh for same reason, however, need use thin Stainless with gaskets or will eventually leak causing problems.

lostmind

It's possible the valves are sticking due to old gas varnish. I would try some treatment down the carb intakes while engine is running. Penetrating oil mixed with lacquer thinner,
followed by a little water. Fixed several  this way. They also make aerosol spray cans with 
cleaners.

tallentr

I think lostmind is on the right track with gummy from the gas. The modern fuel goes bad real quick so I guess running carb cleaner like seafoam thru carb will probably clean up the inside of carbs.

cheyenne7271

Look at the casting plugs under each carburetor on the bottom side of the intake manifold, 3 per side, those get rusty and soft and could be causing a vacuum leak